Generally, conventional solid rocket motors are comprised of a propellant system, a casing, and a nozzle. Solid rocket motors have been made by curing rocket propellant in bulk and then loading it into a rocket casing. More typically, solid rocket motors are made by mixing the propellant components including binder (which is a fuel), oxidizer, fuel (e.g. aluminum) and other additives, and loading (casting) the mixture into a rocket motor casing wherein the "green" propellant mixture is then cured in-situ. Normally the entire loading and curing process is conducted under high vacuum to eliminate air entrapment which would cause propellant faults called "voids".
Specifically, the propellant composition is typically mixed in high shear mixers or extruders such as those used in the bread-making industry. The semi-fluid mixed materials are cast into a motor casing, generally under vacuum conditions until the casing is full. The casing typically will have been prepared prior to casting, by the application of a system of liners (bonding agents) and an overcoat of an insulation material of suitable thickness. Heat is normally used to initiate and control the propellant cure phenomenon resulting in the material becoming a semi-elastic solid.
During cure, the mixture will shrink, which can cause a tensile/shear stress across the bond area, thus creating a problem that often leads to bond failure. Also, this same stress condition tends to magnify as the assembly cool after curing. This condition is magnified at the inner surface of the propellant, frequently enough to cause propellant cracking to occur. The quality of the finished propellant/casing assembly is dependent upon the exclusion of entrapped air; upon the integrity of the propellant to insulation/liner bond; and upon the absence of cracks in the propellant surface. Under operating (burning) conditions, no significant excess propellant burning surface area is permitted such as that which would result in a debond area or around an air-inclusion or that which would be exposed as a propellant surface crack. Following propellant cure, a nozzle is normally attached to finish the assembly.
Although this has been the rocket industry standard for many years, a variety of problems can result (e.g., shrinkage of the propellant mixture during cure, resulting in bond failure to the rocket insulation and casing). Bond failure and/or excessive voids can and often does cause solid rocket motor malfunction that results in explosion or casing burn through, both of which results in loss of motor, vehicle, and mission.
Accordingly, there has been a constant search in this field of art for different methods of making propellants and solid rocket motors that are not subject to such flaws.